Method and apparatus for the refining of aluminum



Dec. 15, 1964 METHOD AND APPARATUS FOR THE REFINING OF ALUMINUM F. W. SOUTHAM Filed Jan. 2, 1962 United States Patent O 3,161,501 ltETHl) AND APPARATUS FR THE REFINING GF ALUMINUM Frederick William Southern, Arvida, Quebec, Canada, assigner to Aluminium Laboratories Limited, Montreal,

Quebec, Canada, a corporation of Canada Filed Z, 1%2, Ser. No. 163,625

8 Claims. (Cl. 75-68) This invention relates to aluminum. More particularly, this invention relates to the rening of aluminum. Still more particularly, this invention relates to the recovery of aluminum from aluminum-containing alloys.

Various processes have been proposed for the rening of aluminum. One process known as the subhalide or catalytic process involves the reaction between solid, particle-form aluminum-containing alloy and a gaseous aluminum trihalide. This process is carried out at a relatively high temperature, usually at a temperature in the range l200-1300 C., more or less. In the relining of aluminum by the so-called subhalide process aluminum-containing alloy, such as a carbothermic aluminum alloy produced by the direct reduction of aluminous material, eg. alumina, with coke, is contacted at a relatively high temperature, about l300 C., with a gaseous aluminum trihalide, such as aluminum trichloride. During the high temperature contacting operation the gaseous aluminum trihalide reacts with the aluminum in the aluminum-containing alloy to form the corresponding gaseous aluminum monohalide, such as aluminum monochloride. The resulting formed gaseous aluminum monohalide is then withdrawn from the reaction or contacting zone and reduced in temperature. Upon reduction in temperature the hot gaseous aluminum monohalide undergoes dissociation with the resulting formation ot substantially pure aluminum and the corresponding aluminum trihalide. The thus-produced elemental aluminum is recovered as product and the resulting formed aluminum trihalide is recovered and returned to the reaction zone to contact additional aluminum-containing alloy.

In the subhalide process for the refining of aluminum, a downwardly moving compact mass of particle-form aluminum-containing alloy is countercurrently contacted as a compact mass with an upwardly flowing stream of hot gaseous aluminum trihalide, such as aluminum trichloride. in this process the temperature of the aluminum-containing alloy within the contacting or reacting zone is a very important operating variable, particularly since the compressive strength of a highly extracted or depleted aluminum alloy, that is, an aluminum alloy having a reduced aluminum content, decreases markedly with an increase in temperature. As a result ol' this loss of compressive strength as the aluminum is extracted from the aluminum-containing alloy during the contacting operation, the reaction or contacting temperature is limited to not more than about 1300 C. On the other hand, fresh, relatively unextracted, aluminum-containing alloy is comparatively strong, even at a high temperature such as 1350 C. Accordingly the top of the reaction Zone or converter containing the, as yet, substantially unextracted aluminum alloy could be operated at a temperature of about 1350o C. or higher, much above the temperature of the bottom portion of the reaction zone or converter containing the aluminum-depleted alloy.

In the subhalide process for the recovery of substantially pure aluminum from an aluminum-containing alloy, heat for maintaining the aluminum-alloy at the reaction ternperature is supplied by means of an electric current flowing through the mass of aluminum-containing alloy within the contacting or reaction Zone. As current flows through the mass of alloy, heat is generated ICC therein due to the resistance of the alloy to the flow of electrical current.

A conventional converter employed in the subhalide process for the rening of aluminum utilizes an upper and a lower set of electrodes. In the subhalide process the power density, usually expressed as kw./ft.3, in conjunction with the gas composition in contact with the alloy determines the alloy temperature. The power density in the upper portion of a conventional converter is only slightly higher in the upper portion of the column or reaction zone than in the lower portion of the column or reaction zone and, because of mass transfer effects, the temperature at the top of the converter or column of aluminum alloy undergoing extraction is lower or at least no higher than the temperature at the bottom portion of the column. In a conventional converter employ'- ing two sets of electrodes, an upper and a lower, the current densities at the top or upper portion and the bottom or lower portion of the charge within the converter, are inversely proportional to the relative crosssectional areas within the converter. Since the electrical resistivity of the aluminum alloy undergoing treatment is relatively unaliected by extraction, i.e. reduction in aluminum content, and the temperature Variation through the converter is not great, the power densities are also approximately inversely proportional to the areas.

It would be very desirable to increase the upper column temperature from about l300 C. to about l350 C. while at the same time maintaining the lower column temperature at about ()D C. By increasing the upper portion column temperature to about l350 C. while maintaining the lower portion of the column a temperature of about 1300 C. an increase in productivity of about 60% is possible for a given converter under substantially the same other operating conditions. This increase in productivity could be obtained with no increase in aluminum trihalide gas ow through the converter and with a very minor increase in pressure drop therethrough. The alternative to this method of increasing productivity involves increasing the gas ow of aluminum trihalide through the converter by about 60%. This, however, results in a higher pressure at the bottom of the converter which, in turn causes an increase in alloy temperature at the bottom of the converter with attendant possible operating diiculties.

In a conventional converter the power density might range from about 3.3 kW./ft.3 alloy charge at the bottom to about 5 kim/ft.3 at the top. In the operation of a conventional converter if a power density of about 1 kw./ft.3 of charge is applied an aluminum extraction rate of about l lb. Al/hrJft.3 will result. For this to happen the temperature of the charge alloy must adjust itself to effect this rate of mass transfer from alloy to gas. Since the rate of chemical reaction is very rapid, the controlling rate in the overall process is the rate of mass transfer. The mass transfer rate is a function of (l) the pressure and composition of the gas exterior of the alloy undergoing extraction and (2) the unextracted alloy composition, local degree of extraction and temperature.

In a given converter operating with fixed outlet conversion and xed degree of extraction, an increase in production can be brought about by increasing the power and increasing the gas flow proportional to the desired increase in production. The alloy compositions and gas compositions with the new production rate will be unchanged at any particular level within the converter, neglecting the effect of changes in pressure drop since the outlet conversion and degree of extraction are unchanged. However, the temperature will have to rise to supply suiiicient driving force to obtain the required mass transfer rate.

Production Rate 2 lbs./t.5/hr. 41bs./it.S/hr. 8 lbs./ft.3/hr.

Temperature at top, C 1, 260 1, 270 1, 280 Temperature down 1, 265 1, 280 1, 310 Temperature 2% down 1, 290 1, 330 1 390 Temperature at bottom 1, 300 1, 350 1, 420

The above data illustrates that by increasing production by increasing gas dow, a marked, undesirable increase in alloy temperature occurs at the bottom of the converter where the alloy is subjected to the greatest compression stresses.

It is accordingly an object of this invention to provide an improved` process for the rening of aluminum.

Another object oi this invention is to provide an improved process for the recovery of aluminum from aluminum-containing alloys, particularly carbothermic aluminum-containing alloys.

Another object of this invention is to provide an improved converter or reactor suitable for use in the subhalide process for the refining of aluminum.

Still another object of this invention is to provide an improved process and apparatus for increasing the productivity of a converter employed in the sub-halide process for the refining of aluminum.

How these and other objects of this invention are accomplished will become apparent in the light of the accompanying disclosure made with reference to the accompanying drawing wherein there is illustrated in vertical cross section a converter suitable for use in the sub-halide process for the rening ot aluminum.

In accordance with this invention it has now been discovered that an improved process for the rening of aluminum, or for the recovery of aluminum from an aluminum-containing alloy, wherein a mass of aluminumcontaining alloy is subjected to high temperature contact with a gaseous stream of aluminum trihalide, is provided by maintaining the upper portion ofthe mass of aluminumcontaining alloy during the contacting operation at a temperature greater than that of the lower portion of the mass of aluminum-containing alloy.

More particularly, in accordance with this invention an improved process for the relining of aluminum is provided yby counter-currently contacting a downwardly moving, compact mass of particle-form aluminum-containing alloy with a gaseous stream containing aluminum trihalide, such as aluminum trichloride to etect reaction between the aluminum in the alloy with the gaseous aluminum trihalide to form the corresponding gaseous aluminum monohalide in accordance with the chemical equation:

wherein X is a halogen atom such as chlorine, bromine or iodine. The contacting operation is carried out at an elevated temperature, such as at a temperature above about 1200 C., preferably above about 1300o C., such as a temperature in the range 1250-l350 C., more or less. During the contacting operation, and in accordance with the practices of this invention, the upper portion of the downwardly moving, compact permeable mass of particleform aluminum-containing alloy is maintained at a higher temperature, such as a temperature in the range of 5-75 degrees centigrade higher, e.g. -50 degrees centigrade, than the lower portion of the mass of particle-form aluminum alloy wherein the alloy is initially contacted with the gaseous aluminum trihalide. By maintaining the upper portion of the mass of alloy, such as a columnar mass of alloy, at a higher temperature than the lower portion during the contacting operation the productivity of the reiining operation or the rate at which aluminum as corresponding aluminum monohalide is produced and recovered from the converter or reactor is greatly increased.

Any suitable means and/or method for maintaining the upper portion of the mass of aluminum alloy at a higher temperature than the iower portion may be employed in the practice of this invention. It is preferred in the practice of this invention to employ electrical heating means to maintain the mass of aluminum alloy at the desired contacting or reaction temperature and to maintain and/ or bring about during the contacting operation a higher temperature at the upper portion than that at the lower portion of the mass of aluminum alloy undergoing contacting.

n a conventional converter employed in the sub-halide process for the refining of aluminum, two electrodes or two sets of electrodes, provided with the necessary attendant electrical `connections and equipment for generating a voltage differential and for causing current to ow therebetween, are provided. As current is caused to flow between the two electrodes, one positioned at the top and the other at the bottom of the converter, through the mass of particle form aluminum alloy in electrical contact therewith, the resistance offered by the alloy to the flow of current therethrough causes the mass of alloy to be heated to the desired reaction or contacting ternperature. After the desired reaction temperature has been reached and while current is owing through the mass of alloy between the electrodes hot gaseous aluminum trihalide, such as aluminum trichlcride, is introduced at the bottom of the converter. Also hot gaseous reaction products comprising unreacted aluminum trihalide and product aluminum monohaiide is withdrawn from the top of the converter. Fresh aluminum alloy is substantially continuously introduced into the top of the converter and spent, aluminum-depleted alloy is withdrawn from the bottom ofthe converter.

An improved converter is provided and an improved method exhibiting an increased productivity under a given set of operating conditions is provided in accordance with this invention by employing a intermediate electrode or set of electrodes between the upper and lower electrodes thereby atiording means for maintaining the temperature of the alloy at the top of the converter higher than the temperature of the alloy at the bottom of the converter. By employing an intermediate electrode the power density, therefore the energy input, in the upper portion of the converter can readily be maintained substantially greater than the power density, energy input, in the lower portion of the converter. For example in a typical converter operated in a conventional manner under substantially isothermal conditions the power density at the bottom would be about 3.3 kw./ft.3 and at the top about 5 kw./ft.3. With an intermediate electrode in accordance with this invention the power density at the bottom might be in the range 2-3.3 kw./ft.3 and at the top at least as high as about 8 kw./ft.3.

In a converter constructed and operated in accordance with this invention the productivity thereof, compared with a conventionally constructed and operated converter, can be substantially increased While at the same time maintaining the same gas ow therethrough as in a conventional converter, and the same degree of extraction, but increasing the power density in the upper part of the converter. For example, if the outlet conversion is increased trom 0.25 to 0.40 and the inlet gas flow is kept constant, the productivity of the converter, thus operated in accordance with this invention, is increased 60%. This means the temperature at the top of the converter, for

lvarying production rates of refined aluminum would be as follows:

At the same time it is necessary to increase the power density over a substantial portion of the converter in the upper portion. In the practice of this invention employing an intermediate electrode it is desirable that this electrode be positioned relative the upper and lower electrodes such that the power density within the converter is about doubled in the zone within the converter equivalent to -50-60% extraction, i.e. from the top of the converter at the point of fresh alloy charge to that location within the converter wherein the downwardly moving alloy charge therein has been depleted of about 5060% of its initial aluminum content. By way of example, operating a converter at an extraction rate of 2 lbs./ft.3/hr. below the intermediate electrode and 4 lbs./ft.3/hr. above it, the temperatures in the converter would vary from 13 10 C. at the top to 1300 C. at the bottom. Higher and lower, top and bottom operating temperatures are possible, depending upon the production rate desired and the composition and physical properties of the alloy undergoing extraction for the recovery of the aluminum therefrom. Operating at a top temperature of 1350D C. would give a very substantial increase in productivity of the converter and could result in an average production of over l0 lbs./ft.3/hr. Operating the converter in accordance with this invention to yield a production rate not greater than about 68 lbs./ft.3/hr. in the upper portion is preferred.

Referring now to the drawing which somewhat sch..- matically illustrates a converter constructed and adapted in accordance with this invention, the converter, generally indicated by the reference numeral 1f) is a vertical, substantially columnar, elongated structure lined with suitable refractory material 11. The converter is provided at the top with an opening 12 for receiving charge alloy and an opening 14 also at the top for the discharge of the gaseous reaction products, mixture of hot gaseous, aluminum monohalide and aluminum trihalide, e.g.

AlCl-i-AlClg via conduit 15 for further treatment as in a decomposer for the recovery of refined aluminum therefrom.

The converter at about the bottom is provided with a circular annular .feed inlet 16 for the introduction of hot gaseous aluminum trihalide, eg. AlCl3, the L,gaseous aluminum tr-ihalide being supplied to feed inlet 16 from `a suitable source, not shown, via conduit 18. At the bottom of lconverter 10 there is provided a conicallyshaped solids discharge device or plug 19 fixed to and rotatable with shaft 20. The speed of rotation of shaft 20 and plug 19 controls the rate of discharge of spent, extracted alloy from the bottom of converter 10 via discharge conduit 21 which communicates with circular bottom discharge converter opening 22. For reasons of better operating efficiency Iand heat economy it is desirable that at least a portion of the gaseous aluminum trihalide supplied to converter 10 pass in direct countercnrrent contact 4with the hot spent extracted alloy as it leaves the converter. This is effected, as illustrated in the drawing, by supplying aluminum trihalide from a suitable source, not shown, via conduit 24 to discharge conduit 21 to contact the hot spent alloy therein before the aluminum halide enters the bottom of the converter.

6 Converter 10 is also provided with an upper set of graphite electrodes 25 mounted so as to be flush with the interior surface of refractory lining 11 and to be in electrical contact with the charge aluminum alloy 26. The converter is also provided with a bottom set of graphite electrodes 28, similarly mounted and disposed as electrodes 25 and also, in accordance with this invention, with an intermediate set of graphite electrodes 29, similarly mounted and disposed las electrodes 25 and 2S. As suggested in the drawing the sets of electrodes 25, 2S and 29 comprise an array, 90 apart, of four electrodes each. In actual practice each set of electrodes, upper, bottom and intermediate, may comprise one or more electrodes which may be plug-type in shape, as illustrated, or ring-type, annular in shape. Desirably, each `set of electrodes is symmetrically disposed within the converter with respect to itself and to the other electrodes. Although only graphite has -been mentioned as the material of construction for the electrodes other suitable, electrically conductive material may be employed. Suitable means, not shown, are also provided for maintaining a voltage differential between electrodes 28 and 29 and between 29 and 25 to cause current to tiow therebetween, more current flowing between electrodes 25-29 than between electrodes 29 and 28.

In the operation of the converter illustrated in the drawing, fresh charge alloy is supplied to converter 10 via inlet 12 to substantially fill the interior thereof with a compact, permeable mass of particle-form charge alloy 26. Hot gaseous aluminum trichloride is introduced into the bottom of the converter via conduit 18 and inlet 16 and hot gaseous reaction products comprising aluminum trichloride and aluminum monochloride are removed from the top of the converter via outlet 14 and conduit 15 Upon establishment of substantially equilibrium operating conditions within the converter, fresh charge alloy is continuously supplied to the upper portion thereof via inlet 12 and hot spent, extracted alloy is continuously discharged from the bottom thereof via outlet 22 and discharge conduit 21. Some heat recovery is effected from the discharged hot spent alloy by direct countercurrent heat exchange between the hot spent alloy and supplemental, relatively cool aluminum trichloride introduced into the bottom of the conventer via conduit 24 and spent alloy outlet 22.

The desired operating temperatures are maintained within the mass of charge alloy 26 within converter 10 by causing current to flow between the bottom electrodes 28 and intermediate electrodes 29 and between intermediate electrodes 29 and top electrodes 25. The heat required to yield desired operating temperatures is effected by the How of current through the compact mass of alloy 26. Due to the electrical resistance of the mass of alloy to the flow of current therethrough the various desired operating temperatures within the mass of alloy 26 within converter 10 are readily obtained by controlling or adjusting the flow of current through the various portions, lower and upper, of the mass of alloy within converter 10. In a typical operation the current density in the lower portion of the charge alloy is smaller than that of the upper portion, e.g. current ow, measured as power density, might range from 1-4 kwa/ft.3 at the bottom to 5-10 kws./ft.3 at the top, to yield temperatures in the range about 1250-1280 C., more lor less, at the bottom to a temperature in the range about I300-1350" C., more or less, at the top.

As will be apparent to those skilled in the art in the light of the foregoing disclosures, many modifications, alterations and substitutions 4are possible in the practice of this invention without departing from the spirit or scope thereof.

Iclaim:

1. A method which comprises introducing particleform aluminum-containing alloy into the top of a substantially vertical converter and withdrawing spent particle-form alloy from the bottom of said converter to provide a compact, downwardly moving mass of particleform aluminum-containing alloy within said converter, introducing `gaseous aluminum trihalide into the bottom of said converter, withdrawing a gaseous admixture comprising aluminum monohalide and aluminum trihalide from the top of said converter and, during the aforesaid aluminum tri'halide contacting operation carried out at an elevated temperature of at least approximately 1200 `degrees centigrade whereby reaction is ef- :fected between the aluminum in said aluminum-containing alloy and the ygaseous aluminum trihalide to yield gaseous aluminum monohalide, causing electrical current to ow through the mass of particle-form aluminum alloy within said converter to provide heat to yield said elevated temperature such that electrical current flows through the upper portion of said mass of alloy within said converter at a higher rate than flows through the lower Iportion of said mass of alloy within said converter to maintain -said upper portion of said mass of alloy at a temperature in the range -75 degrees centigrade higher than the temperature of said lower portion of said mass of alloy.

2. A method in accordance with claim l wherein said upper portion of said mass of alloy is maintained at a temperature of about 1350J C. and said lower portion of said mass of alloy is maintained at a temperature of about 1300 C.

3. A method in accordance with claim 1 wherein said aluminum trihalide is aluminum trichloride.

4. A method in accordance with claim 1 wherein said aluminum trihalide is aluminum trichloride and wherein said upper portion of said mass of alloy is maintained at a temperature of about 1350 C. and said lower portion of said mass of 4alloy is maintained at a temperature of about l300 C.

5. In an operation wherein a substantially vertical columnar mass of particle-form aluminum-containing alloy is contacted at an elevated temperature of at least approximately l200 C. within a reaction zone with a flowing stream of gaseous aluminum trihalide to effect reaction between -said particle-form alloy and said aluminum trihalide to yield the corresponding gaseous aluminum monohalide, said mass of alloy within said reaction zone being maintained at the aforesaid elevated temperature by causing electrical current to ow between two electrodes, an upper electrode and a lower electrode, disposed along the length of and in electrical contact with said columnar mass, the space between said upper and said lower electrodes defining said reaction zone and the resistance offered by said alloy within said reaction Zone to the liow of current therethrough supplying the heat to maintain said alloy within said reaction zone at said elevated temperature, the improvement in combination therewith which comprises disposing an intermediate electrode between said upper electrode and said lower electrode in electrical Contact with said mass of alloy within said reaction zone, that portion of said reaction Zone between said upper electrode and said intermediate electrode being dened as the upper portion of said reaction zone and that portion of said reaction zone between said intermediate electrode and said lower electrode being defined as the lower portion of said reaction Zone, and causing electrical current to ilow between said intermediate electrode and said upper electrode at a greater rate than ows between said intermediate electrode and said lower electrode so as to maintain the mass of alloy within the upper portion of said reaction zone at a temperature in the range 5-75 degrees centigrade higher than the temperaturehof the mass of alloy within the lower portion of said reaction zone.

6. A method in accordance with claim 5 wherein said gaseous aluminum trihalide is aluminum trichlorrde,

7. A method in accordance with claim 5 wherein sald mass of particle-form aluminum-containing alloy comprises carbothermic aluminum alloy.

8. A method in accordance with claim 5 wherein aluminum initially comprises 35-85% by weight of said particle-form aluminum-containing alloy.

References Cited in the file of this patent UNTTED STATES PATENTS 757,633 Price Apr. 19, 1904 935,344 White Sept. 2S, 1909 2,270,245 Barker Jan. 20, 1942 2,723,911 Phillips et al. Nov. l5, 1955 2,937,082 Johnston et al. May 17, 1960 

1. A METHOD WHICH COMPRISES INTRODUCING PARTICLEFORM ALUMINUM-CONTAINING ALLOY INTO THE TOP OF A SUBSTANTIALLY VERTICAL CONVERTER AND WITHDRAWING SPENT PARTICLE-FORM ALLOY FROM THE BOTTOM OF SAID CONVERTER TO PROVIDE A COMPACT, DOWNWARDLY MOVING MASS OF PARTICLEFORM ALUMINUM-CONTAINING ALLOY WITHIN SAID CONVERTER, INTRODUCING GASEOUS ALUMINUM TRIHALIDE INTO THE BOTTOM OF SAID CONVERTER, WITHDRAWING A GASEOUS ADMIXTURE COMPRISING ALUMINUM MONOHALIDE AND ALUMINUM TRIHALIDE FROM THE TOP OF SAID CONVERTER AND, DURING THE AFORESAID ALUMINUM TRHALIDE CONTACTING OPERATION CARRIED OUT AT AN ELEVATED TEMPERATURE OF AT LEAST APPROXIMATELY 1200 DEGREES CENTIGRADE WHEREBY REACTION IS EFFECTED BETWEEN THE ALUMINUM IN SAID ALUMINUM-CONTAINING ALLOY AND THE GASEOUS ALUMINUM TRIHALIDE TO YIELD GASEOUS ALUMINUM MONOHALIDE, CAUSIGN ELECTRICAL CURRENT TO FLOW THROUGH THE MASS OF PARTICLE-FORM ALUMINUM ALLOY WITHIN SAID CONVERTER TO PROVIDE HEAT TO YIELD SAID ELEVATED TEMPERATURE SUCH THAT ELECTRICAL CURRENT FLOWS THROUGH THE UPPER PORTION OF SAID MASS OF ALLOY WITHIN SAID CONVERTER AT A HIGHER RATE THAN FLOWS THROUGH THE LOWER PORTION OF SAID MASS OF ALOY WITHIN SAID CONVERTER TO MAINTAIN SAID UPPER PORTION OF SAID MASS OF ALLOY AT A TEMPERATURE IN THE RANGE 5-75 DEGREES CENTIGRADE HIGHER THAN THE TEMPERATURE OF SAID LOWER PORTION OF SAID MASS OF ALLOY. 